1. Field of the Invention
The invention relates to a waveguide filter for circuits operating at microwave frequencies.
2. Description of the Related Art
Within the context of broad bandwidth wireless networks, it is known to use increasingly high frequencies so to allow the transmission of high bit-rate applications such as video. The purpose of these networks is also to allow data exchange between at least one base station (access providers) and a plurality of user stations, networks commonly called point-multipoint networks. At present, the standard MPT-1560-RA provides for the use of frequencies located between 40.5 and 42.5 GHz.
In order to carry out bidirectional communications of the full-duplex type, it is known to allocate frequencies which are different for transmission and reception. However, it is necessary to have recourse to high-performance filtering devices in order to separate the transmission and reception signals, since the transmission signal interferes with the reception signal. In order to meet the filtering restrictions (microwave, broad bandwidth, high rejection outside the bandwidth), it is known to have recourse to filters with cavities coupled by discontinuities, commonly called waveguide filters.
Waveguide filters act on the received electromagnetic wave before the latter is transformed into an electrical signal. The filtering function is provided by the shape of the structure. The size of the filter is proportional to the wavelength, the length of the cavities being equal to half of the wavelength.
One technique for fabricating a waveguide filter consists in producing two shells, each equivalent to half of the filter cut along a plane passing through its central axis, then in reassembling the two shells. In order to work with millimeter waves, it is necessary to have good accuracy, to less than 50 xcexcm on the finished filter. The production of a filter in two shells with such accuracy involves a production cost which is too high for it to be integrated into a mass-produced product. There are other techniques, in particular using tuning screws to compensate for tolerance drift, the fabrication costs of which are also high.
In addition, documents GB-A-0 731 498 and DE-A-35 12 936 disclose waveguide filter devices as shown in FIG. 1. The filter consists of a plurality of discs 2 and 3 held in a support 1. A first type of disc 3 defines the sizes of the filter cavities, and a second type of disc 2 defines the cavity separations. Such a filter is difficult to isolate electrically when the number of discs is high.
The invention proposes a waveguide filter with a low-cost structure. The filter of the invention consists of a plurality of discs held in a cylinder with a square, rectangular or circular base. In the present document, the term cylinder must be understood by the person skilled in the art as the mathematical definition, namely a volume generated, on the one hand, by a straight line which is displaced parallel to a fixed direction while standing on a fixed planar curve and, on the other hand, two parallel planes cutting the generating straight lines, the projection of the planar curve on one of the planes in the fixed direction corresponding to the base of the cylinder. Each disc defines an iris and a cavity, which reduces the number of electrical contacts. The various parts constituting the filter are simple and therefore cheap, while providing good conductivity due to a smaller number of contacts than in the prior art.
Thus, the invention is a waveguide filter which comprises a support comprising a cavity, the cavity being a cylinder of any base, and a plurality of discs the external shape of which corresponds to the base of the cylinder, each disc comprising at least one recess whose depth corresponds to the length of a cavity, the end of the recess being furnished with at least one coupling aperture. The filter thus produced has the other advantage of not requiring any tuning during assembly.